The present invention relates to a cathode ray tube, and particularly to a cathode ray tube having its overall length shortened with its deflection angle increased, but without increasing deflection power consumption or degrading display resolution.
Cathode ray tubes such as color cathode ray tubes used as TV picture tubes and monitor tubes for information terminals house an electron gun for emitting a plurality (usually three) of electron beams at one end of an evacuated envelope, a phosphor screen (a viewing screen) formed of phosphors coated on an inner surface of the evacuated envelope at the other end thereof for emitting light of a plurality (usually three) of colors, and a shadow mask which serves as a color selection electrode and is closely spaced from the phosphor screen.
The electron beams emitted from the electron gun are deflected to scan the phosphor screen horizontally and vertically in two dimensions, by magnetic fields generated by a deflection yoke mounted externally of the evacuated envelope and display a desired image on the phosphor screen.
FIG. 16 is a schematic cross-sectional view of a shadow mask type color cathode ray tube as an example of a cathode ray tube to which the present invention is applicable, and FIG. 17 is a front view of a panel portion of the color cathode ray tube of FIG. 16.
In FIG. 16, reference numeral 1 denotes the panel portion forming a viewing screen, 2 is a neck portion, 3 is a funnel portion, 4 is a phosphor screen, 5 is a shadow mask, 6 is a mask frame, 7 is a magnetic shield, 8 is a mask suspension mechanism, 9 is an in-line type electron gun, 10 is a deflection yoke, 11 is an internal conductive coating, 12 is a shield cup, 13 is a contact spring, 14 is a getter, 15 is a stem, 16 are stem pins, 17 is an implosion protection band, 18 is a magnetic beam adjusting device, and 19 is a usable display area.
In FIG. 16, a dimension L is a distance from the phosphor screen 4 to the end of the anode on the focus electrode side thereof, of the in-line beam type electron gun 9, and a dimension d is an outside diameter of the neck portion 2. In FIG. 17, a dimension D is a diagonal length of the usable display area 19.
The evacuated envelope of this color cathode ray tube is comprised of the panel portion 1, the neck portion 2 and the funnel portion 3. Three electron beams (one center electron beam Bc and two side electron beams Bs) emitted from the in-line type electron gun housed in the neck portion 2 is scanned over the phosphor screen 4 two-dimensionally by the horizontal and vertical deflection magnetic fields generated by the deflection yoke 10 mounted around the transition region between the funnel portion 3 and the neck portion.
The highest voltage (an anode voltage) to the electron gun is supplied by the contact springs 13 attached to the shield cup 12 via the internal conductive coating 11 coated on the inner surface of the funnel portion 3 from an anode button (not shown) embedded in a wall of the funnel portion 3.
The deflection yoke 10 is of a self-converging type which provides a pin cushion-like horizontal deflection magnetic field and a barrel-like vertical deflection magnetic field to converge a plurality of electron beams over the entire phosphor screen.
The electron beams Bc, Bs are modulated in amount by modulating signals such as video signals supplied via the stem pins 16, are color-selected by the shadow mask 5 disposed immediately in front of the phosphor screen 4, and impinge upon the phosphors of the corresponding colors to reproduce a desired image. Color purity of the reproduced color image and static convergence of the three electron beams are adjusted by the magnetic beam adjusting device 18 mounted around the neck portion 2.
In color cathode ray tubes of this type, a large-diameter non-axially-symmetric lens formed between an anode and a focus electrode are extensively used as a main lens system of the electron gun to provide sufficiently small electron beam spots over the entire phosphor screen.
FIG. 18 is a schematic side elevation view of a prior art electron gun employing the large-diameter non-axially-symmetric lens system viewed in a direction perpendicular to the in-line direction of the electron beams. In this electron gun, an electron beam generating section is comprised of a cathode 21, the first grid electrode 22 and the second grid electrode 23, and an accelerating and focusing section is comprised of the third grid electrode 24 serving as a focus electrode and the fourth electrode 25 serving as an anode. The cathode and electrodes are fixed on a pair of insulating rods 26 made of glass in the predetermined order and the predetermined spaced relationship.
The contact springs 13 are attached to the front end of the shield cup 12 which in turn is attached to the anode 25. The highest voltage is applied to the anode 15 by the resilient contact springs 13 pressed against the internal conductive coating 11 on the inner wall of the funnel portion 3.
FIG. 19 is a plan view of the third grid electrode 24 viewed from an anode side thereof and FIG. 20 is a cross-sectional side view of the third grid electrode 24 viewed in a direction perpendicular to the in-line direction of the three electron beams. Reference 31 denotes an electric field correction plate having three vertically elongated electron beam apertures with their minor diameters in the in-line direction of the electron beams and disposed within the third grid electrode 24, and reference numeral 32 denotes an electrode having the configuration of the outer periphery of a racetrack shape (hereinafter referred to as a racetrack electrode) and formed with a single opening with its major diameter in the in-line direction of the electron beams.
FIG. 21 is a plan view of the anode 25 viewed from the third grid electrode 24 side thereof and FIG. 22 is a cross-sectional side view of the anode 25 viewed in the direction perpendicular to the in-line direction of the three electron beams. Reference 33 denotes an electric field correction plate having a vertically elongated electron beam aperture at the center with its minor diameter in the in-line direction of the electron beams and cutouts on opposite sides of the electron beam aperture and disposed within the anode 25, and reference numeral 34 denotes a racetrack electrode formed with a single opening with its major diameter in the in-line direction of the electron beams. With such an electrode structure, an effectively large-diameter electron lens is formed between the grid electrode 24 and the anode to provide a high definition image display.